High Pressure and High Temperature Annealing of Ni-Containing, Nitrogen-rich Synthetic Diamonds and the Formation of NE8 Centers

Emerging quantum technologies require the fabrication of diamonds that contain single defects with specific optical, electronic, and magnetic properties. We report on the results of an annealing study performed at a pressure of 3.5 GPa with a temperature range of 1500–1900 °C on Ni-containing, nitrogen-rich (up to 640 ppm) synthetic diamonds and the formation of NE8 (N₂NiN₂) centers. This work examines the nitrogen-vacancy defects and nickel-related defects in detail. The results shown that high nitrogen concentration is more conducive to the formation of high-intensity NE8 centers compared with diamonds having a low nitrogen content. The aggregation of the A-center nitrogen facilitated the formation of the NE8 center. Furthermore, the intensity of the negatively charged nitrogen-vacancy (NV^–) center decreased at higher annealing temperatures in the range of 1500–1800 °C. Our experimental results help increase the understanding of the formation of various centers in diamonds and their associated relationships to realize the effective control of the NE8 center concentration.